New To MAC World & RAID 5 ?'s

Hello to all and thanks for the help in advance.

I just got a MacPro 2.8GHz (8-core). I would like to add three 2TB drives in RAID-5 Fashion to protect my data.

1st? Can I do this with Software instead of the card? What software

2nd? If I decided to add an additional drive in the optical bay at a later time can I do that or would everything need formatted

3rd? Can you incorporate RAID 5 with both external & internal harddrives (all same size) together or do the have to be separate or can externals not be done in RAID-5

I am trying to decide which direction to go I bought 3 2tb Seagate USB3.0/Firewire 800 drives before I decided on MAC Pro instead of MacMini Server and am looking at what options I have both now and in the future.